A number of countries has legislation containing maximum limits for the emission of nitrogen oxides (NOX) in exhaust gases of vehicles driven by combustion engines. A method to reduce the amount of NOX in the exhaust is the use of selective catalytic reduction (SCR). In SCR systems, NOX in the exhaust gas are converted via a reductive chemical reaction into nitrogen and water. The conversion involves a catalytic reaction using a reductor. Ammonia is frequently used as reductor. The ammonia is obtained by disintegration of a precursor solution containing ammonia or urea. The precursor liquid is injected in the exhaust gas flow. To this end a car has one or more tanks containing the precursor liquid. An example of such precursor liquid is AdBlue (trademark of VDA, Verband der Automobilindustrie), commercial name of a water based solution containing 32.5% urea. Other examples are urea/ammonia formate solutions (e.g. sold under the commercial name Denoxium). Such precursor solutions are very corrosive.
Most water based precursor solutions can freeze at temperatures that occur in nature (e.g. a water based solution of 32.5% urea freezes at minus 11° C.). Therefore a heating element is required to thaw the precursor tank when the precursor liquid in the tank is frozen. The heating element can also be used to heat the precursor liquid to a suitable operational temperature.
Heating systems to be immersed in SCR tanks have to answer a number of requirements. A first requirement is that the heating systems must be able to generate a sufficient amount of heat in order to thaw and/or heat the required amount of liquid in a sufficiently short time period. The required amount of heat depends on the volume of the tank, and in particular on the maximum amount of precursor liquid the tank can contain. The dimensions of the heating element have to be limited to leave sufficient space in the tank for the tank to contain a sufficient volume of precursor liquid without the tank and the accessories in and around the tank becoming to voluminous. It is requested that heating is performed first around the pump that is pumping the precursor liquid out of the tank, but other sections of the tank also have to be thawed and heated. Preferably, the heating system can heat different sections of the tank (e.g. remote sections in the tank or sections that are physically separated from other sections of the tank). Thawing should not only be performed around the pump, but also in other sections, such that precursor liquid is available at and towards the pump. The tank can have a complex design, with different accessories provided in the tank (pumps, sensors . . . ). Heating systems immersed in the tank are very efficient in terms of transfer of energy (heat) to the (frozen) precursor liquid. However, the immersed heating system should also be resistant to the conditions in the tank: the corrosive precursor (or volatile components, such as ammonia) and temperatures that in use can go up to over 100° C. Required is a long life time of the heating element, without change over time of its characteristics (including generation of heat). Electrical (resistive) heating systems have to be energy efficient, and have to be able to operate without demanding (too) large amounts of energy from the battery of the vehicle, which is especially of importance when starting the vehicle in cold weather conditions. Manufactures of SCR systems prefer heating systems that are easy to install.
A number of different systems have been developed to thaw and heat the tank that is containing the precursor liquid. Some systems comprise separate resistive heating elements or a bypass of hot liquid or of hot gas around or through the tank. In general the heating systems are complex and expensive.
WO2008/138960 describes a urea tank and a base plate with an integrated heating element wherein the heating element comprises at least one flexible part that preferably consists out of an electrical resistance track fixed onto a flexible film and/or placed between two flexible film layers. The complex composition of heating is a drawback and—although flexible films are used—a further drawback is the rather high stiffness of the of the heating element. The latter is making it more difficult to install the heating element in the tank, especially in remote sections of the tank. No liquid flow is possible through the films, limiting the capacity of fluid flow in the tank. Liquid flow in the tank is favorable to equalize the temperature in the tank and to increase the efficiency of the thawing process.
EP-A1-2216524 is disclosing a urea strainer that is including a mesh for removing impurities from urea passing therethrough and heatable wires to heat the urea as it passes through the mesh. The urea is provided to an emission control system of a diesel engine. It is a disadvantage of the heating element that only local heating in the tank is possible.